A source list is an area of window set off by a movable pane splitter to provide users a way to navigate data. Use a source list when the data presented in it is a primary means of navigating within the application, as in iTunes or the Finder. Users select objects in the source list that they act on in the main part of the window.

And source lists are apparently the reason that the Panther Finder’s windows are (sometimes) metal:

You can use a brushed metal window if your application:

Provides an interface for a digital peripheral, such as a camera, or an interface for managing data shared with digital peripherals—iPhoto or iSync, for example.

Strives to re-create a familiar physical device—Calculator or DVD Player, for example.

Provides a source list to navigate information—for example, iTunes or the Finder.

Don’t use the brushed metal look indiscriminately. Although it works well for some types of applications, some applications appear too heavy when using this look. For example, it works well for the iSync application window, but it does not work very well for the TextEdit document window.

Of course, by this logic, Xcode and System Profiler windows would be metal, Safari browser windows would change to metal only when you viewed the bookmarks, the Safari Downloads window wouldn’t be metal, and on and on. It’s not worth thinking about this too deeply, because the brushed metal guidelines are more a retroactive justification for Apple’s design fetishes than a set of sound principles for designing usable, consistent interfaces.

Also note that they’re now called “brushed metal windows” rather than “textured windows.” Hopefully this is meant as a reassurance that the texture won’t be changed to Blue Dalmation in Mac OS X 10.4.

Will you people get over the brushed metal look. It is so obvious why Apple decided on this go with this look. In Classic OS 9, Apple used a gray scheme because its neutral and easy on the eyes. With OS X, they got rid of this scheme and everything was all white and was so bright. If you work all day staring at the monitor, this becomes annoying. Apple discovered the error of their ways and brought back some gray but updated it a bit to make it look modern....thus the brushed metal look. Its a lot better to look at compared to the painfully all-white theme previously in Jaguar and earlier versions of OS X. My guess is they will introduce more grays in future updates. OS X is still too white for my taste. OS 9 might be an outdated OS but the UI was well-developed.

Sy: the problem I have is that some apps are metal and others aren't. Further, if you use Unsanity's haxie to make every app metal, you'll see that this looks way too "heavy" (to use Apple's term). So, if the brightness is a problem, and I think you may be right about that, Apple should come up with a slightly darker Aqua theme that can work across the board.

Sy: your comments don't make any sense. The brushed metal window appearance was first seen in the classic Mac OS with QuickTime 4, Mac OS 8.6!!! It definitly wasn't invented as a way to tone-down Mac OS X's excessive brightness problem.

Hai. If Aqua is too bright, then they should simply provide a darker theme. If Metal is an attempt to darken aqua, the result is that the brightness of the user interface is continually changing. If metal is an attempt to differentiate between types of applications, then all windows of a Metal application should be metal... and Terminal.app should be metal because it's used to interface to hardware. Then there's the whole Finder shenanigans...

I agree with "the brushed metal guidelines are more a retroactive justification for Apple’s design fetishes than a set of sound principles for designing usable, consistent interfaces".

I think Apple also uses the brushed metal to make the software more visually in sync with their (future) hardware.
Which are generally either white or metal.
In my opinion this adds "usability" because the software refers to "real-world".
I was surprised by the Garage Band window having wooden sides though.

Also the visuals of the OS are constantly being updated and modified, it seems to be a work in progress.
In that situation you can't stick to guidelines too strictly simply because they might have changed between updates.

In my house I move stuff around and throw things out, I like it that Apple does the same on their OS :)